Specifications

cells has to be erased first to convert all cells to ones; then the particular cell has to be written again to turn

it to a zero. In order to write to a NAND memory chip, the controller must follow a 2-step process:

1. Erase all the cells in a block to ones.

2. Write zeroes to particular cells in a block in order to store digital data.

Flash memory is divided into blocks that are generally 128 kB in size, each block holding 64 pages of 2 kB.

“Random” writing in NAND memory means:

1. Erasing an area for the new data for a block or a series of blocks

2. Copying all the unchanging data into the new area

3. Writing the new zeroes into the same area.

Even if only one 2 kB-page changes, the entire block of 128 kB has to be rewritten; and the writing of that

block is sequential, one page at a time throughout the entire block. That is why random access writing takes

so much longer than simple sequential writing. It is also why speed ratings for flash cards and USB flash

devices apply only to the faster sequential write speeds than the much slower random write speeds.

FLASH CARD READERS

Every computer today still comes with a floppy disk drive. The drives and the diskettes are

inexpensive and very handy; but as computer files grow ever larger, the limited formatted* capacity

of 1.44MB becomes a handicap. Flash media are much handier, faster, and more reliable in

addition to being able to hold at least 7 times more information. There are two ways to transfer

data to and from a computer via flash cards: 1) using a PCMCIA card slot (common among

laptops, unusual for desktops), or 2) using a flash card reader attached to a USB port on a desktop

or laptop.

PCMCIA Adapter

Many laptops have PC Card PCMCIA card slots installed in them for these older and larger

storage cards. Adapters allow the newer flash cards to fit into them so that the newer cards are

fully functional when the adapter is plugged into the PCMCIA slot. There are also other types of

adapters on the market for Smart Media, Secure Digital, and Memory Stick cards. The

PCMCIA/Compact Flash combination is an easier and more economical method of transferring

large files to and from a laptop than adding a CD-R/-RW drive to the computer if a floppy disk drive

is no longer adequate to hold today’s larger files.

*Flash media require formatting just as floppy disks, CD-RWs, and DVD recordable discs used for storing

files. The format defines the address structure on the medium so that files can be erased, moved, or altered

without disrupting the other information that is stored. Most flash media and floppy discs have the formats

installed during verification testing during the manufacturing process. Optical media do not come pre-

formatted (except for some format information molded into the surface of DVD-RAM discs) because the

process would take too long. Formatting takes up storage space on a rewritable medium so that the full

stated capacity is more than actual storage capacity. A double-sided, double density floppy disk has a

stated capacity of 2.0MB, but only 1.44MB after formatting. A 700MB CD-RW has about 550MB of storage

capacity after formatting. A 128MB flash card can have 122MB of storage capacity after it is formatted.

Improper use of a flash card can corrupt the data on the card so that a digital camera, for example, will show

error messages for the card. The cause is most often interruption of the write cycle due to turning the

camera off too soon or pulling the card out before writing was finished. Other causes can be a battery that is